Process of and apparatus for photographically recording sounds



June 8, 1937.

R. M. OTIS PROCESS 0F AND APPARATUS FOR PHOTOGBAPHICALLY RECORDINGSOUNDS s Sheets-Sheet 1 Filed July' 17, 1929 Tidy/ a //v'1/E/v Toe:EussELL M June 8, 1937. 2,082,846

PROCESS ow AND APPARATUS FOR PHOTOGRAPHICALLY RECORDLNGYSOUNDS 'R. M.OTIS Filed July 17, 1929 s sn ets-sneei 2 //v l/EN TOE E066ELL I! June8, 1937. R 01-15 2,082,846

PROCESS OFAND APPARATUS FOR PHOTOGRAPHICALLYRECORDING SOUNDS Filed July17, 1929 3 Sheets-Sheet 3 I I I 1 [NZ/51V Toe: I IUSSELL M 0776.

Patented June 8, 1937 PROCESS OF AND APPARATUS FOR PHOTO- GRAPHIC'ALLYRECORDING SOUNDS Russell M. Otis, Alhambra, Calif.

Application July 17,

22 Claims.

My invention relates to a novel system for indicating or recordingvarious phenomena, and more particularly to a novel method of recordingsounds on a motion-picture film or other 5 photo-sensitive element, aswell as to novel apparatus for accomplishing this result.

If a pair of electrodes are placed in close contact with opposite sidesof a photographic plate, or other photo-sensitive element, and a momen-10 tary high potential impressed thereacross, as by the discharging of aLeyden jar therethrough, the photo-sensitive emulsion will be exposed toform a Lichtenberg figure. The shape or" this figure is dependent uponthe shape of the elec- 15 trodes. For instance, the use of cylindricalelectrodes produces a Lichtenberg figure composed of a large number oflines extending radially outward from the area of contact between theelectrode and the photo-sensitive element. Furthermore, my experimentshave shown that Lichtenberg figures may be formed in the area of contactbetween the electrode. and the photo-sensitive element if the voltagegradient is sufficiently high.

I have found it possible to confine a Lichtenberg figure to an area ofone one-thousandth of an inch or less in width and of a length equal toor somewhat greater than the width of the space on a motion-picture filmallotted to a sound track, this being accomplished by suitably designingthe electrodes and by utilizing means for preventing uncontrolledfringing of the discharge taking place between the electrodes.

It is an object of this invention to provide a method of and apparatusfor utilizing Lichtenberg figures in the production of talkingmotionpictures, and in various other recording and indicating arts.

A further object of the invention lies in a novel apparatus and methodfor confining the discharge forming a Lichtenberg figure to an area ofone-thousandth of an inch or less in width, thus making my methodapplicable to the recording of sound on a motion-picture film 45travelling at the customary speed.

At present sound records of the variable density or variable area typesare formed by complex light valves, vibrating mirrors, or lamps ofvarying intensity, all of these systems being subject 50 to burn-outsand most systems being affected by vibration to such an extent thatportable use of the apparatus is impractical.

It is an object of this invention to provide an apparatus for recordingsounds or other phe- 55 nomena which includes no moving parts, no parts1929, Serial N0. 378,994

which will burn out, and which requires no optical system.

A further object of the invention is to provide an extremely simplemethod of sound-recording which requires no special equipment notreadily available to the industry.

Another object of my invention is to provide an apparatus which willform either a sound record of the variable density type, or of a typeintermediate between the variable density and variable area types. Thisnew intermediate type of sound record has images thereon which fade orbecome less dense in a direction transverse to the sound track, as wellas in the direction of motion of the film, thus forming a record whichis in some instances very desirable.

It is a further object of this invention to provide a motion-picturefilm having a sound track containing photographic images which fade in agiven direction.

These and other objects I accomplish by moving a motion-picture film orother photo-sensitive element between a pair of electrodes suitablydesigned, and impressing on these electrodes a high potential suitablymodulated in accordance with the undulations of the sounds to berecorded or in accordance with the other phenomena to be recorded.

It is an object of this invention to form a sound record on amotion-picture film by the use of a high potential modulated by theundulations of the sound to be recorded.

Still another object of the invention is to use potentials of highfrequency in the photographic recording of sounds, thus eliminating thedanger of serious shocks and materially decreasing the magnitude of thepotentials required.

Another object of the invention is to modulate a high potential, whetheror not this potential is of high frequency, with impulses of anotherfrequency to change either the magnitude, wave form, or frequency of thehigh potential, and to utilize the modulated potential in the formationof Lichtenberg figures.

Yet another object of this invention is to provide a novel knife-edgeelectrode and mounting structure therefor, as well as a method of makingthis electrode structure.

Further objects and advantages of my invention will be evident to thoseskilled in the art.

Referring to the drawings,

Fig. l is a diagrammatic face view of one form of my recording apparatusin combination with a section of motion-picture film.

Fig. 2 is a sectional view taken on the line 2-2 of Fig. 1.

Figs. 3 and 4 are alternative forms of the device as illustrated in Fig.1.

Fig. 5 is still another form of the invention.

Fig. 6 is a view partially in section taken in the direction of thearrow 6 of Fig. 5.

Fig. '7 is a view partially sectioned showing one of the electrodes ofmy invention in the form of a roller incorporated with a sprocket.

Fig. 8 is a view similar to Fig. 2 illustrating another form of theinvention.

Fig. 9 is a wiring diagram illustrating one method of connecting myinvention for the purpose of recording sounds.

Fig. 10 is a graph illustrating a modulated highfrequency potential.

Fig. 11 is one type of sound record recorded by my apparatus.

Fig. 12 is another form of sound record which may be. recorded.

Fig. 13 is one form of apparatus for recording a sound record such asshown in Fig. 12.

Fig. 14 is another form of apparatus for recording a sound record suchas shown in Fig. 12.

Referring particularly to Figs. 1 and 2, my invention in its simplestform utilizes primary and secondary electrodes iii and l l spaced asufiicient distance. apart to allow a motion-picture film I2 to passtherebetween. The secondary or lower electrode is shown as comprising ametallic plate of a width substantially wider than the sound track ofthe film l2, and of any suitable length. This secondary electrode maytake various different forms as will be hereinafter explained.

The primary electrode shown in Figs. 1 and 2 is made of a very thinpiece of metallic foil, the thickness thereof being preferably oneonethousandth of an inch or less, this foil being made of a conductingmaterial having a high melting point such as platinum iridium, as bestshown in Fig. 1. This piece of foil is substantially rectangular withits lower corners rounded to prevent a concentration of the dischargethereadjacent. The. lower surface of this electrode proi ldes aknife-edge, and it is desirable that'the field intensity between thisknife-edge and the electrode ll be substantially constant throughout thelength of the knife-edge or throughout that portion of the knife-edgewhich covers the sound track.

The thinness of this electrode necessitates external support which isfurnished by a pair of blocks l5 and 56, these blocks being of a widthgreater than the width of the electrode l9, and of a height slightlyless than that of the electrode to permit easy attachment of a conductorl8 leading to a modulating device ii? of invention. the other terminalof this device being connected by a conductor 26 to the electrode ll.

Adjacent surfaces ill and 22 of the blocks 55 and 16 are carefullysurfaced so that all portions of each face lie in the same plane. Theseblocks are preferably formed of opaque glass though other insulatingsubstances may be used, and the surfacing of the adjacent faces 2i and22 may be accomplished by grinding, being careful to maintain the edgesof the blocks intact. Lower faces 23 and 2d of these blocks are alsoaccurately ground so as to lie in a common plane when the adjacentsurfaces 2! and 22 are in contact.

he foil forming the electrode ill is positioned adjacent the surfaces 2!and 22 and clamped or cemented therebetween, the lower edge thereofextending slightly below the lower faces 23 and 24, the electrode l andinsulating blocks i and it thus forming an electrode structure 25 whichin itself forms a part of my invention.

Subsequently, the protruding portion of the foil is carefully removed bygrinding or other means, and the lower faces 23 and 24 repolished toinsure a continuous plane surface devoid of depressions, the objectbeing to form a continuous flat surface which may contact the film, thissurface including a thin edge of conducting material acting as anelectrode. By carefully forming this electrode structure fringing of theLichtenberg figure can be eliminated to such an extent that it does notnoticeably increase the effective thickness of the line formed.

It is also possible to form a thin deposit of metal on one of theadjacent surfaces 2| and 22 by means known in the art, this depositbeing used as an electrode.

The electrode structure 25 may be loosely mounted so that the weightthereof is eifective in insuring good surface contact with the film E2,or the electrodes may be moved resiliently together by means of suitablesprings acting either upon the lower or upper electrode structures asillustrated respectively in Figs. 3 and 4.

In Fig. 3, I have illustrated an alternative form of secondary electrodewhich is in the form of a bar 33 having rounded upper edges 3! toprevent a brush discharge therefrom, and being adapted to expose a soundtrack of the film l2, the extent of this sound track being indicated bythe double-headed arrow 32, it being noted that the width of this soundtrack is somewhat less than the length of the lower electrode 3 due tothe rounded edges of this electrode. A spring 33 may resiliently movethe bar 35 into engagement with the lower surface of the film !2.

Likewise, in Fig. l, I have shown a bar 35 similar to the bar 3t exceptthat it is rigidly mounted and includes a coating of insulating material36 which further prevents any extraneous discharge therefrom. In thisform springs 3'! bear against the electrode structure 25 and resilientlymove this structure toward the electrode formed by the bar 35 in amanner to lightly compress the film l2 therebetween.

In some instances it is desirable that the secondary electrode H be inthe form of a roller so as to provide rolling contact with the film, theline of contact therebetween being directly below the knife-edge formedby the electrode I0. Thus, in Figs. 5 and 6, I have illustrated a roller45 as serving this function, this roller having a shaft ti journalled inbearings 32 to which the conductor 29 is connected. Each bearing 42 hasa square end portion 33 which is vertically slidable in an elongatedopening 44 of a standard 45, the bearings 42 being urged upward relativeto the standard by means of springs 46 compressed therebetween. Thewidth of the roller ill in this instance is slightly greater than thewidth of the sound track, the width of which is indicated by the arrows32. This is due to the rounded edges which are usually necessary on theroller 46.

Fig. 7 illustrates a roller 48 serving as a secondary electrode andbeing mounted in a sprocket 69 formed of insulating material andproviding sprocket teeth 5% for driving the film. The roller 48 is alsorounded in this form of the invention, and the insulated sprocket G9 isso formed as to correspond to these rounded edges. The sprocket may beformed by a pair of members 5| and 52 clamped on a shaft 53 formedintegrally with or modulated,

for compressing the members 5| and 52 against.

the roller 48, but it is preferably formed by molding the roller 48 andshaft 53 into a dielectric such as bakelite. 1.

Fig. 8 illustrates still another combination of electrodes wherein boththe primary and secondary electrodes are of similar construction andbeing formed in the manner previously described for the electrodestructure 25.

I have found it desirable to prevent brush discharge from theelectrodes, this being accom plished by rounding the edges thereof asprevi ously described. Rounding the edges of a thin piece of foil is,however, very difficult, so that I have found it best practice to makethe foil electrode of greater length than the space allotted to thesound track, and utilize the lower electrode for determining the lengthof image formed. It is comparatively easy to round the edges of thislower electrode as indicated, for instance, in Figs. 3, 4, 5, 6, and 7in such a manner as to insure a uniform field and prevent pointdischarge.

It is thus apparent that the width of the instantaneous image formed atany instant is lim-- ited by the blocks [5 and it, while the length ofsuch an instantaneous image is determined by the shape of the lowerelectrode. So also the shape of the discharge which takes place from thelower edge of the primary electrode I0 is limited by these blocks and bythe shape of the secondary electrode ll.

Themodulating device I9 is of such character as to supply to theelectrodes 10 and l I a potential modulated by the undulations of thesounds to be recorded. The amount of exposure of the film may be variedby (1) changing the amplitude or magnitude of the impressed potential(2) changing the wave form of this impressed potential, or (3) changingthe frequency thereof. It should thus be clear that I amusing the termmodulated in a broad sense to include any of these means for varying anexposure, and do not intend this term to be limited to the narrowermeanings sometimes allotted thereto wherein only the magnitude of thepotential is varied. Thus, in the claims I have used the termssoundmodulated, etc., to include a change in amplitude, wave shape, orfrequency of a potential of such magnitude as to be capable of exposingthe photographic emulsion on the film I2, or to include a simultaneouschange of two or more of these quantities.

The most convenient and simplest system for use in recording sounds isone in which a high frequency potential is modulated by lowerfrequencies in the audible range. This may most conveniently byaccomplished by the system illustrated in Fig. 9 wherein an oscillatortube Bil is connected to a tuned circuit 6| in a manner well-knownwhereby oscillations are set up therein. The tuned circuit Si in Fig. 9includes a condenser 62 and an inductance 63 which also serves as partof a Winding of an auto-transformer 64 to which the conductors l8 and 20connect.

The high frequencies impressed on the tuned circuit 5! by the oscillatortube 65 may be modulated in any well-known manner by impulses of lowerfrequency corresponding to the undulations of the sounds to be recorded.In Fig 9 the modulation system includes a modulator tube 85 having aplate supply tl common to that of the oscillator tube 68, there being aniron core inductance 68 between the supply 61 and the modulator tubewhereby changes in plate ourrent of the modulator tube change thevoltage drop across the inductance 68, thus varying the potentialapplied to the plate of the oscillator tube 60 and thereby modulatingthe high frequency currents in the tuned circuit 5| in amannerwell-known to the art when voice frequencies are impressed on thegrid of the modulator tube66 by means of a transmitter or microphone 69connected thereto through the usual transformer 10 or other couplingdevice.

This modulating circuit is well-known, and no claim is made to thiscircuit per se aside from the particular method of connecting theautotransformer 64.

Furthermore, while I have shown the coupling device in the form of anauto-transformer this use is not essential for other coupling devices,such, for instance, as an ordinary transformer of suitable design may beused.

The advantages of such a system are mani- .fold. In the first place, theoscillatorand moducycles per second. Lower. frequencies may be used solong as they are out of the audible range. Other sources of potentialmay be utilized, such, for instance, as static machines or high voltagegenerators or magnetos, but I have found it preferabl e to use highfrequencies, not only from the standpoint of convenience, but primarilybecause the potential necessary to form Lichtenberg figures is muchlower when high frequencies are used than when low frequencies are used,the ratio being in the neighborhood of one to ten. Furthermore, byutilizing high frequencies modulated by audible frequencies all dangerof serious injury to an operator is obviated. It is wellknown thatmanual contact with a circuit carrying high voltages of high frequencyproduces no. serious results, though contact with a circuit of equalvoltage of low frequency might cause instant death.

It is also 'much simpler to generate a high frequency current by meanssuch as shown in Fig. 9, for instance, than to produce highdirectcurrent voltages which may be modulated in response to audiofrequency impulses. Similarly, systems wherein the oscillation frequencyis varied in response to audio frequency impulses ordinarily are morecomplicated than that shown in Fig. 9, as are also systems wherein thewave form is so varied by audio frequency impulses as to change theexposure of the film in response to such impulses. All of these systems,however, fall within the scope of my invention.

It is sometimes desirable to render the modulating device l9 inoperativein the event that the movement of the film l2 ceases, thus preventingany possibility of burning or puncturing the film. This may beaccomplished by a centrifugal switch connected in an appropriate placein the circuit. In Fig. 9, I have shown a switch 12 in the plate circuitof the tubes 60 and 65, this switch being opened by a governor 73 whenthe film I2 stops. The governor may be suitably connected by a shaft 15to a sprocket 76 which drives the film.

My experiments have shown that if the po' tential of the high frequencycurrent is accidentally increased to such a point that disruptivedischarge takes place through the film no harm to the film or apparatusresults. While such a disruptive discharge punctures the film the filmdoes not catch fire, and the discharge is quickly stopped.

The type of record produced by the apparatus previously described isdiagrammatically illustrated in Fig. 11, and is known as the variabledensity type due to its having a series of lines extending betweenboundary lines 85 and 8| of a sound track 32. This type of record isproduced by a modulated high frequency wave as shown diagrammatically inFig. 10, it being understood that the modulation shown is notnecessarily one that would be effected by modulation with a complexvocal sound. Degrees of modulation other than that shown in Fig. l0 mayalso be used.

In Fig. 12, I have illustrated another form of sound track wherein eachline varies in intensity between the boundary lines as and Bi, being ofgreater intensity adjacent the base line 80. These lines are not of thesame length but each one gradually fades out at a point in the soundtrack depending upon the potential, frequency, and wave form, as well ason other conditions. The result of such a series of lines isdiagrammatically shown in Fig. 12 by lin s of varying thickness, fromwhich it is apparent that the images not only fade in the direction ofthe movement of film, indicated by the arrow 83, but also fade in adirection perpendicular thereto, thus forming a sound record which has'very desirable characteristics in certain types of recording.

The type of record illustrated diagrammatically in Fig. 12 may be formedby the apparatus illustrated in Figs. 13 or 14. Referring particularlyto Fig. 13, I have illustrated the primary electrode as being formed ofa coating 89 of highresistance material. A piece of high-resistance foilmay be utilized in some cases in place of this coating 89 or a depositof a material such as selenium may form this coating. Certain finelyground stones may also be used, usually in combination with a suitablebinder which, if desired, may in itself have high-resistance properties.Other substance which may be utilized include finely divided graphite,carbon, etc. usually intermixed with a suitable binder.

A pair of the insulating blocks previously described is used to supportthe high-resistance electrode and to limit the discharge to a narrowarea corresponding. in magnitude to the thickness of the electrode 89.In this form of iy invention the conductor l8, previously described, isconnected to one end of the primary electrode and the other end of thishigh-re sistance electrode is connected by a wire so to a secondaryelectrode ill which is formed of good conducting material and which isconnected by the wire 2? to the modulating device IS. The secondaryelectrode 9! may be of any one of the forms previously described.

Inasmuch as the upper electrode is made of high-resistance material, itshould be apparent that the current passing therethrough causes avoltage drop in this electrode, the difference in potential between theprimary'and secondary electrodes being thus a function of the distanceacross the sound track. In other words, the

potential difference between the electrodes at a section A--A wouldapproach zero, due to the fact that the wire 96 short-circuits this endof the primary electrode with the secondary electrode. At a section BB,however, full potential is established, and the lines formed at thissection will be relatively intense, andwill form Lichtenberg figureswhich are dense as indicated at the base line 80 in Fig. 12. At anintermediate section CC the potential difference between the primary andsecondary electrodes is, of course, less than that at section BB by anamount depending upon the ratio of the horizontal distance between thesections 3-3 and CC relative to the distance between the sections AA andBB.

Thus, when using a modulated high frequency such as shown in Fig. 10,the lines formed will extend variable distances across the sound trackdue to the fact that the voltage gradient in the gap between theelectrodes varies as previously explained.

Fig. 14 illustrates another pair of electrodes for accomplishing thisresult. In this figure the primary electrode is formed of a goodconducting material, indicated by the numeral 95, and is mounted betweeninsulating blocks as previously described, the only difference betweenFig. 14 and Fig. l, for instance, being that the electrode 85 of Fig. 14has a lower edge which is not parallel to the surface of the film, thiselectrode being cut to form an angled surface Q6, one end of which liesin the same plane as the surfaces 23 and 2d of the insulating blocks l5and I6, and the other end lies a distance thereabove so as to form a gaptl between the knife-edge and the surface of the film, this gap being ofvariable height throughout the width of the electrode 95.

This gap may be left open to the air, or may be filled with otherdielectric. Any difference in potential between the electrodes which issufficient to produce Lichtenberg figures will register on the soundtrack at a point adjacent the heel of the electrode 95, and the figureproduced will be less dense toward the toe of the electrode due to thevarying distances across the gap 97.

It is within the scope of my invention to introduce amplifiers eitherbetween the micro phone or the modulator or between the oscillator tubeand the recording device, or in other places in the event that suchamplification is desired.

The time interval required to produce an exposure by my method is veryminute, being in the neighborhood of one ten-millionth of a sec- 0nd.This quick action is very desirable in the recording of sounds, inasmuchas my recording system will not vibrate at a natural period in theaudible range nor in resonance with sounds in the audible range as docertain other devices at present in use. The extreme sensitivity andspeed of the device of my invention is such as to make it desirable inrecording high frequency, transient, and other phenomena, my inventionbeing adaptable to these other uses by only slight changes, such, forinstance, as the speed of the film and a substitution of the circuit tobe tested for the oscillatory circuit to be hereinafter described.

It will be apparent to those skilled in the art that my invention is notlimited to the field of sound-recording, but is also applicable to theindication or recor dation of various other phenomena. For this reasonmy invention should not be limited to the use of a motion-picture film,

it being that the Lichtenberg figures may be formed on any suitablephoto-sensitive surface such as might be present on a photo-sensitiveelement such as a photographic plate or on a piece of photographicpaper.

It is also possible that it may be subsequently shown that thephenomenon which produces Lichtenberg figures is not an actinic one, butthat the exposure of the photo-sensitive emulsion is due to othereffects present in the discharge. In such an event my invention wouldobviously include other substances on which Lichtenberg figures might beformed, the essence of my invention being the subjection of a film orother substance to a variable discharge. In these other arts it isfurthermore not necessary that the oscillating frequency be of themagnitude de scribed, nor is it necessary that the modulating frequencybe within the audible range. Thus, in certain of the appended claims Ihave spoken of the primary potential as being modulated by a secondarypotential, my invention also finding utility with such a combinationwhether or not one of the potentials is a unidirectional one.

In the appended claims I have used the term discharge not in the senseof a disruptive discharge, but rather such a discharge as is sufficientto produce photographic exposure whether this exposure be due to abombarding stream of electrons or positive ions or whether due to X-raysor actinic ionization of the air immediately adjacent the surface of thefilm. Similarly, I have used the term Lichtenberg figures and the termimages to not only desighate the figures and images as they appear afterdevelopment, but also the exposure which may be subsequently developedinto a visible Lichtenberg figure or image.

I claim as my invention:

1. A method of producing talking motionpictures, which method includesthe steps of: forming photographic Lichtenberg figures on amotion-picture film varying in density in response to variations ofsound to be recorded; and confining said Lichtenberg figures so thatthey are of substantiallyconstant area and of a width of one-thousandthof an inch or less.

2. A method of recording sound on a photosensitive. surface, whichincludes the steps of providing an electrostatic field of suflicientmagnitude to form photographic Lichtenberg figures and influenced inintensity by the audio frequency sounds to be recorded; moving said filmthrough said field; and confining to lineal dimensions the individualLichtenberg figures formed at any instant of time.

3. In combination: a primary knife-edge electrode positioned on one sideof a photo-sensitive element; a secondary electrode on the other side ofsaid photo-sensitive element; means for subjecting the portion of saidelement lying between said electrodes to an electric stress suficient toform a photographic Lichtenberg figure on said element; and means forconfining said photographic Lichtenberg figure to an area notsubstantially greater in width than the thickness of said knife-edgeelectrode at the point of discharge.

4. In an apparatus for recording sounds on a motion-picture filmincluding an unexposed sound track, the combination of a primaryelectrode providing a knife-edge of length greater than the width ofsaid sound track; a secondary electrode positioned on the opposite sideof said film from said primary electrode; and means for impressing asufficient potential across said electrodes to form a photographicLichtenberg figure on said film, said secondary electrode being soformed as to limit said photographic Lichtenberg figure to a length notmaterially greater than the width of said sound track.

5. An electrode structure, comprising: a pair of supporting blocksformed of insulating material and having adjacent surfaces; and anextremely thin electrode plate formed of material more conducting thansaid insulating material and positioned between said adjacent surfacesand terminating in a knife edge bounded on opposite sides by saidsupporting blocks.

6. An electrode structure, comprising: an electrode providing a knifeedge and being so thin as to require external support; and supportingmeans on opposite sides of said electrode and providing adjacentsurfaces engaging faces of said electrode and between which saidelectrode is supported, said supporting means providing a plane surfacelying in the same plane as that end of said electrode forming said knifeedge, said surface providing said plane being surfaced to eliminatelarge pockets adjacent said knife edge.

7. An electrode as defined in claim 5 in which said electrode is formedof an electrically conducting substance and is less than onethousandthof an inch in thickness.

8. In a recording system, the combination of a primary electrode in theform of an extremely thin plate providing a knife edge; a secondaryelectrode spaced from said knife edge, there being a photo-sensitiveelement therebetween; means for impressing a potential between saidelectrodes sufficient to form a discharge which is recorded on saidphoto-sensitive element; and a supporting structure for said primaryelectrode and terminating in the plane including said knife edge toprevent spreading of said discharge.

9. A combination as defined in claim 8 in which said supportingstructure comprises a pair of blocks formed of insulating material andretaining said electrode therebetween, said blocks contacting saidphoto-sensitive element.

10. In a recording system, the combination of: a primary electrode inthe form of an extremely thin plate providing a knife edge; a secondaryelectrode spaced from said knife edge, there being a photo-sensitiveelement therebetween; means for impressing a potential between saidelectrodes sufiicient to form an image on said photo-sensitive elementimmediately beneath said knife edge; and a supporting structure for saidsecondary electrode, said supporting structure lying adjacent saidsecondary electrode and engaging said photo-sensitive element.

11. A combination as defined in claim 10 in which said secondaryelectrode provides rounded edges, and in which said supporting structureconforms to said rounded edges whereby those portions of said secondaryelectrode and said supporting structure which engage said photosensitiveelement lie in the same plane.

12. In a sound recording system, the combination of: a primary electrodein the form of an extremely thin plate providing a knife edge; asecondary electrode spaced from said knife edge, there being aphoto-sensitive element therebetween; means for impressing a potentialbetween said electrodes sufficient to form a discharge which is recordedon said photo-sensitive element at a section immediately beneath saidknife edge; means for varying said potential in response to theundulations of the sound to be recorded; and means for rotatablysupporting said secondary electrode.

13. A combination as defined in claim 10 in which said secondaryelectrode and said supporting structure therefor comprise a sprocket forsaid photo-sensitive element.

14. In a recording system for recording on a moving photo-sensitiveelement, the combination of: a primary knife-edge electrode on one sideof said photo-sensitive element; a secondary knife-edge electrode on theother side of said photo-sensitive element, the knife edges of saidelectrodes being parallel and disposed opposite each other; and meansfor impressing a varying difference in potential between said electrodessufiicient to expose that portion of said photosensitive element lyingbetween said electrodes.

15. In a recording system for recording sounds on a motion-picture filmhaving a sound track 1 of a limited width, the combination of: a primaryknife-edge electrode extending across said sound track and substantiallyin contact therewith for a distance greater than the width of said soundtrack; a secondary electrode on the other side of said film from saidprimary electrode and substantially engaging said film for a distancesubstantially equal to said width of said sound track whereby the fieldset up between said electrodes is of a width substantially equal to thewidth of said sound track; means for impressing a potential between saidelectrodes sufficient to expose said film; and means for limiting thearea of exposure of said film to substantially that area'of said soundtrack contacted by said primary electrode.

16. In a sound-recording system, the combination of a primary electrodein the form of a plate and having a thickness less than oneone-thousandth of an inch; a pair of supporting blocks, one beingpositioned on each side of said electrode; a secondary electrode spacedfrom said primary electrode; means for moving a photosensitive elementbetween said electrodes; meansfor impressing a difference of potentialbetween said electrodes sufiicient to expose said photosensitiveelement; and means for varying said difference in potential in responseto the sound to be recorded.

17. An electrode structure, comprising: a flat electrode terminating ina knife edge; and means engaging said electrode and confining anyelectric discharge from said knife edge to an area immediately beneathsaid knife edge, said means comprising a structure snugly engaging thesides of said electrode adjacent said knife edge to eliminate all airpockets and spaces into which the discharge from said knife edgecouldspread.

18. A method of recording sounds on a motionpicture film, which includesthe steps of: moving said film; forming an electric discharge directedthrough said moving film and of a shape approaching lineal dimensions,said discharge extending transverse with respect to the direction ofmovement of said film; varying the intensity of said discharge inresponse to sound undulations to correspondingly vary the density of theimage produced on said film; and limiting the spread of said dischargealong the surface of said film toform instantaneous images ofsubstantiallyconstant dimensions and of a width not substantiallygreater than one-thousandth of an inch.

19. A method of recording sound one photo-- sensitive element, whichmethod includes the steps of: setting up an electrostatic field entirelythrough said photo-sensitive element and of sufficient intensity to forman image on said photosensitive element; varying said electrostaticfield in response to the undulations of the sound to be recorded;setting up a relative movement between said photo-sensitive element andsaid electrostatic field whereby images are formed on successiveportions of said photo-sensitive element, said images varying in densitywith the variations of said electrostatic field; and confining the imageformed at any particular instant of time to knife-edge proportions ofsubstantially constant length and width.

20. A method of recording on a moving photosensitive element by the useof a pair of electrodespositioned on opposite sides of saidphotosensitive element, which method includes the steps of establishinga potential difference between said electrodes and of sufficientmagnitude to expose said photo-sensitive element to form an imagethereon, but of insufficient magnitude to puncture said photo-sensitiveelement; varying said potential difference in response tothe variationsin the phenomena to be recorded; confining said image to a substantiallyconstant area of knife-edge dimensions regardless of the potentialdifference between said electrodes, whereby the variations in saidpotential produceimages correspondingly varying in density but of'substantially constant area; and producing relative movement betweensaid electrodes and said photo-sensitiveelement.

21. A method of recording on a photo-sensitive element by the use of anelectrode, which method includes the steps of bringing said: electrodeinto surface contact with said photo'- sensitive element; setting upan-electric field adjacent said electrode and through saidphotosensitive element thereby tending to expose said photo-sensitiveelement to form both a: fringing image spreading radially from saidelectrode in the plane of the surface of said photo-sensitive elementand an auxiliary image immediately be neath said electrode andsubstantially covering the surface of contact between said electrode andsaid photo-sensitive element; shielding said electrode to eliminate saidfringing image al-' lowing only said auxiliary image to be impressed onsaid photo-sensitive element whereby said auxiliary image is of an areasubstantially thesame as said areaof contact; moving said photosensitive element through said field; and varying the intensity of saidfield in response to-the phenomena to be recorded whereby the density ofsaid auxiliary image varies correspondingly.

22. A sound recording apparatus consisting ofmeans for converting soundwaves into sound modulated electric current, a pair of contact elementsconnected to said converting means to receive said sound modulatedcurrent, one of said contact elements consisting of an electricallyconducting film moving element, and a movable photo-sensitive surfaceplaced between and-in di-' rect contact with one of said contactelements to be chemically altered thereby to produce a developablelatent sound record by the action of said current upon saidphoto-sensitive surface RUSSELL M. OTIS;

